1. Field of Invention
The current invention relates to emulsions, systems and methods of producing emulsions, and more particularly to systems and methods of producing emulsions having reduced droplet sizes and to emulsions having reduced droplet sizes.
2. Discussion of Related Art
Nanoemulsions are dispersions of metastable droplets of one liquid in another immiscible liquid that have droplet radii a below 100 nm (Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109). They are kinetically inhibited against coalescence by a surfactant that provides a strong stabilizing repulsion between the droplet interfaces. Typically, extreme shear or extensional flow are necessary to create a nanoemulsion, since the viscous stresses, τv, on the droplet's surfaces must overcome the Laplace pressure, ΠL=2σ/α, where σ is the interfacial tension, of spherical parent droplets (Mason, T. G. Curr. Opin. Colloid Interface Sci. 1999, 4, 231). As a result, very high strain rates {dot over (ε)} approaching 108 s−1 are usually necessary to create water-based nanoemulsions (Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109). A strong surfactant and low solubility of the dispersed oil phase in the continuous phase are critical for producing long-lived nanoemulsions that do not coarsen through Ostwald ripening (Durian, D. J.; Weitz, D. A.; Pine, D. J. Science 1991, 252, 686; Gopal, A. D.; Durian, D. J. Phys. Rev. Lett. 2003, 91; Mason, T. G.; Krall, A. H.; Gang, H.; Bibette, J.; Weitz, D. A. Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1996; Vol. 4). (The terms oil phase and continuous phase used herein refer to two immiscible materials that can be used to produce an emulsion. In some embodiments, the continuous phase can be an aqueous material in which oil droplets are dispersed to form an oil-in-water emulsion. In other words, each of the two immiscible materials is sometimes referred to as a “phase” for conciseness.)
Extreme emulsification is typically used to make metastable nanoemulsions (T. G. Mason, J. N. Wilking, K. Meleson, C. B. Chang, and S. M. Graves, Nanoemulsions: Formation, Structure, and Physical Properties, J. Phys.: Condens. Matter 18 R635-R666 (2006)). As opposed to lyotropic microemulsions that are thermodynamic phases comprised of self-assembled nanostructures, nanoemulsions are not equilibrium thermodynamic phases, but instead are out-of-equilibrium dispersions of nanoscale droplets of one liquid in another immiscible liquid. Here, the term nanoscale is used to refer to droplets that have radii when undeformed that are typically less than about 100 nm. Distinguishing characteristics between “nanoemulsions” and “microemulsions” are the following:
nanoemulsionmicroemulsionnon-equilibrium dispersion of dropletsthermodynamic phase of nanostructuresformed by droplet rupturing typicallyformed by self-assemblyextreme flow is typically required to formforms spontaneously-no mixing is requirednanostructures are droplets of a dispersednanostructures can be swollen sphericalphase coated with surfactantmicelles, lamellae, columnar micelles, . . .significant liquid-liquid interfacial tensionvery low liquid-liquid interfacial tensionvery low mutual solubility of immisciblesignificant mutual solubility of immiscibleliquid phasesliquid phasessingle surfactant stabilizes dropleta surfactant and usually a co-surfactantinterfaces against coalescence(e.g. an alcohol) reduce interfacial tensionlittle to no exchange of dispersed phaserapid exchange of dispersed phase betweenbetween dropletsmicellar structures
The process of extreme emulsification has been used to rupture larger emulsion droplets down into nanoscale emulsion droplets by imposing an extreme flow using a high-pressure microfluidic device or an acoustic or ultrasonic device. Strong viscous flows around the larger droplets stretch out the droplets, and an interfacial instability known as the “capillary instability”, driven by the interfacial tension between the two liquid phases, causes the stretched droplets to break up into two or more smaller droplets. This process continues until all of the droplets in the emulsion have effectively been ruptured down to nanoscale dimensions.
Although extreme emulsification can be used to produce oil-in-water nanoemulsions with droplets that have radii, a, as small as about a≈15 nm, typically a significant quantity of surfactant must be used to reach such small sizes (T. G. Mason, J. N. Wilking, K. Meleson, C. B. Chang, and S. M. Graves, Nanoemulsions: Formation, Structure, and Physical Properties, J. Phys.: Condens. Matter 18 R635-R666 (2006)). For lower surfactant concentrations that are more economical, droplet sizes are typically larger, in the range of 40 nm<a<100 nm. It would be useful to have an economical method that could reduce the droplets in a larger nanoemulsion down to much lower nanoscale droplet sizes. Furthermore, the development of a droplet size reduction method would enable the size distribution of the emulsion to be controlled better through the composition. There thus remains a need for improved systems and methods for making emulsions and for improved emulsions.